1. Field of the Invention
The present invention relates to a semiconductor device having a main element and a sense element for detecting current flowing through the main element, and to a current detector circuit in which the semiconductor device is used.
2. Related Art
Power semiconductor elements such as an IGBT (Insulated Gate Bipolar Transistor) or MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) that supply current to a load are such that it is vital to detect current flowing through the power semiconductor element in order to carry out control for supplying an appropriate current to the load and an operation protecting against overcurrent.
In order to do this, frequent use is made of a composite element having an emitter terminal E that supplies current to a load and a sense terminal S that causes current proportional to the current flowing through the emitter terminal to flow (FIG. 4 shows circuit symbols thereof). See, for example, Japanese Patent Application Publication Nos. JP-A-10-32476, JP-A-2010-246179 and JP-A-2012-85407. The composite element will be described with an IGBT as an example, and hereafter, in the same way, an IGBT will be given as an example of a power semiconductor element, but the invention, not being limited to an IGBT, is also applicable to other power semiconductor elements, such as a MOSFET.
A composite element 10 shown in FIG. 4 actually has the configuration shown in FIG. 5. That is, the composite element 10 is formed of a main element 100 and sense element 200, to both of which a collector terminal C and gate terminal G are connected. A sense terminal S corresponds to the emitter terminal of the sense element, and current that is 1/N (N<1) of current flowing through an emitter terminal E (of the main element) flows through the sense terminal S.
A configuration example of the composite element 10 shown in FIGS. 4 and 5 on a semiconductor substrate 300 is shown in FIG. 6. In FIG. 6, unit elements 20 (IGBTs, MOSFETs, or the like, of a minimum size or a size near the minimum) are disposed regularly, one thereof configures the sense element 200, and the other N1 unit elements 20 configure the main element 100. The collector terminal, gate terminal, and source terminal of each of the unit elements 20 configuring the main element 100 are connected to each other by wiring not shown in the drawing. Also, as heretofore described, the gate terminals and collector terminals of the main element 100 and sense element 200 are also connected. Herein, N is basically equal to N1, but it may happen that the two are not of exactly the same value owing to the effect of the element ends.
Also, other than the configuration shown in FIG. 6, the configuration may be such that one large power semiconductor element is divided into two regions of differing sizes, the larger region is taken to be the main element 100 and the smaller region taken to be the sense element 200, and a gate terminal, source terminal, and collector terminal are provided in each thereof (the gate terminal and collector terminal are connected to each other).
An example of a basic configuration of a circuit that carries out control and a protective action with respect to the composite element 10 is shown in FIG. 7. The circuit shown in FIG. 7 has the composite element 10, a resistor Rs (the resistance value thereof is also taken to be Rs), a reference voltage Vref, a comparator 30, and a control circuit 40. 50 is a load driven by the composite element 10. Current flowing through the sense terminal S is converted to a sense voltage Vs equal to Rs×Is by the resistor Rs, and the sense voltage Vs is input into a non-inverting input terminal of the comparator 30. Also, the reference voltage Vref is input into an inverting input terminal of the comparator 30. The comparator 30 carries out a comparison of the sense voltage Vs and reference voltage Vref, and transmits the result of the comparison to the control circuit 40. The control circuit 40 normally controls the turning on and off of the composite element 10 based on a command from the exterior, but when obtaining information from the comparator 30 that the sense voltage Vs is greater than the reference voltage Vref (when the output of the comparator 30 is a an H (High) level), the control circuit 40 determines that a current IE of the main element 100 proportional to a current Is is an overcurrent, and controls a gate voltage Vg of the composite element 10 so as to cause the composite element 10 to be turned off.
Also, FWD is a diode for causing a flow of current in a direction the reverse of that of current (forward current) flowing through the composite element 10. As the composite element 10 can cause only forward current to flow, the composite element 10 and diode FWD are used as a set in almost all applications. Also, a power supply Vi (the voltage thereof is also expressed as Vi) is connected to the collector terminal C of the composite element 10 and to the cathode of the diode FWD.
A timing chart for when the composite element 10 shown in FIG. 7 changes from an off-state to an on-state is shown in FIG. 8. Also, an equivalent circuit of the composite element 10 for illustrating the operation shown in FIG. 8 is shown together with the resistor Rs in FIG. 9. In FIG. 9, Cies1 is an input capacitor of the main element 100 formed of a gate capacitor or the like, and Cies2 is an input capacitor of the sense element 200 formed of a gate capacitor or the like.
FIG. 8 shows, in order from the top, the gate voltage Vg of the composite element 10, collector current Ic and collector voltage Vc, and the sense voltage Vs. Vth in the drawing is the threshold voltage of the composite element 10.
In FIG. 8, the gate voltage Vg of the composite element 10 begins to rise at a time t10, and on the gate voltage Vg reaching the threshold voltage Vth of the composite element 10 at a time t20, the collector current Ic begins to flow, together with which the collector voltage Vc begins to decrease.
After the sense voltage Vs overshoots once, it becomes a steady value.
Basically, the collector current Ic and sense voltage Vs are in a correlative relationship (proportional relationship) but, the time of the gate voltage Vg rise being an exception, it is clear that the overshooting of the sense voltage Vs at the time of the gate voltage Vg rise is a phenomenon peculiar to the sense element 200 that has no connection with the collector current Ic.
As the overshooting of the sense voltage Vs has no connection with the collector current Ic, it is a cause of inhibition with regard to detection of the collector current Ic. The inventor has determined that the overshooting of the sense voltage Vs occurs due to a flow of current in response to variation in the voltage of the gate terminal G in the differential circuit formed of the input capacitor Cies2 and resistor Rs shown in FIG. 9. That is, the waveform of the sense voltage Vs is such that the overshoot caused by the differential circuit is superimposed on the original sense voltage proportional to the collector current Ic, because of which, even when the original sense voltage Vs (the last steady value of FIG. 8) is less than the reference voltage Vref that forms the reference for determining an overcurrent, as shown in FIG. 8, the sense voltage Vs exceeds the reference voltage Vref due to the amount of the overshoot being superimposed, and is erroneously determined to be an overcurrent. An actual product is such that it is difficult in terms of cost to provide the element with large excess power, because of which it is not unusual that the original sense voltage Vs is close to the reference voltage Vref.
False determination of the sense voltage Vs as an overcurrent due to an overshoot, even though it is not an overcurrent, is a problem, because of which means whereby overcurrent detection is prohibited for a certain time from the gate voltage beginning to rise has heretofore been adopted. Also, in See, for example, Japanese Patent Application Publication No. JP-A-7-240516, a sense voltage Vs overshoot causing mechanism differing from that of the invention is envisaged, attempting to prevent overshoot of the sense voltage Vs by causing conductivity of the sense element 200 in the composite element 10 to be later than conductivity of the main element 100. Each of these means has a problem in that an overcurrent cannot be detected without waiting a certain time (dead time), even though current has begun to flow through the main element 100. That is, immediate response is not possible even in the urgent situation of the load short circuiting, which includes the danger of leading to the serious accident of ignition. Also, with the method of Japanese Patent Application Publication No. JP-A-7-240516, it is not possible to prevent the phenomenon of overshoot caused by the differential circuit of the gate input portion from being superimposed on the original sense voltage, because of which the possibility of false detection remains.